The present invention relates to microlithography apparatus and methods using a charged particle beam. With such apparatus and methods, a pattern as defined on a mask or reticle is transferred to a sensitive substrate (e.g., semiconductor wafer) using a charged particle beam (e.g., electron beam or ion beam). The invention more specifically relates to such apparatus and methods in which the velocity of the charged particle beam incident to the sensitive substrate can be made different from the velocity of the charged particle beam incident to the reticle.
In an ongoing effort to develop practical microlithography apparatus that can achieve better resolution than optical microlithography, microlithography using a charged particle beam (e.g., electron beam) has received much attention. However, a practical charged-particle-beam (CPB) microlithography system has not yet been realized due to many technical problems such as satisfactory aberration control and acceptable throughput (number of semiconductor wafers that can be processed microlithographically per unit time).
In a typical CPB microlithography system as exemplified by an electron-beam system, an xe2x80x9cillumination beamxe2x80x9d is produced by an electron gun and passed through multiple condenser lenses (constituting an xe2x80x9cillumination-optical systemxe2x80x9d) to illuminate a region on a reticle. The portion of the illumination beam passing through the reticle becomes a xe2x80x9cpatterned beamxe2x80x9d that passes through multiple projection lenses (constituting a xe2x80x9cprojection-optical systemxe2x80x9d) to form an image, on the wafer, of the illuminated region of the reticle.
Japanese Kokai (laid-open) patent document no. Hei 8-124834 discloses electron-beam microlithography apparatus in which a decelerating electric field is established between the reticle and the wafer (i.e., within the projection-optical system) to reduce the velocity of the charged particle beam incident to the wafer relative to the beam velocity incident to the reticle. Maintaining a relatively high beam velocity at the reticle reportedly yields better contrast and electron-beam transmission through the reticle, even when using a scattering contrast reticle. A relatively high beam velocity at the reticle also reportedly reduces electron absorption by the reticle (which reduces reticle heating due to electron absorption by the reticle) and reduces chromatic aberration. A relatively low beam velocity incident to the wafer reportedly helps prevent loss of resist sensitivity and reduces heating of the resist and of the wafer.
However, apparatus and methods according to JP 8-124834 pose the following problems:
(a) The beam-decelerating electric field is produced by imposing a high voltage to a xe2x80x9cliner tubexe2x80x9d in a lens of the projection-optical system. Application of such a high voltage to the liner tube produces a localized beam-decelerating electric field that produces a corresponding localized electrostatic lens action between the wafer and the liner tube (or between the wafer and a shield of the projection-optical system, as shown in FIG. 2 of that reference). The lens action generates a new aberration for which no corrective action is disclosed or contemplated by the reference.
(b) Application of a high voltage to a liner tube of a lens causes other problems leading to aberrations and beam blur. This reference provides no information on how to solve such problems or how to correct for aberrations arising from irregularities in the surface planarity of the wafer.
In view of the shortcomings of the prior art summarized above, the present invention was devised to achieve one or more of the following:
(1) In a simple manner, control parasitic aberrations arising from passing the charged particle beam through a beam-decelerating electric field.
(2) Provide the charged particle beam with high energy at the reticle using a relatively low-voltage power supply.
(3) Exploit a change in the beam half-angle of the charged particle beam within a projection lens and at the surface of the wafer that arises from subjecting the beam to a beam-decelerating electric field. For a particular beam half-angle, a relatively small beam half-angle in the lens yields reduced geometric aberrations and chromatic aberrations. (The reduction in chromatic aberration is especially pronounced.)
(4) Avoid problems with lenses of the projection-optical system that arise due to application of high voltage to the respective liner tubes.
(5) Achieve improved beam adjustment and registration.
(6) Avoid deleterious effects of a non-planar surface around the reticle and/or wafer.
To achieve the ends listed above, and according to a first aspect of the invention, charged-particle-beam (CPB) microlithography apparatus are provided. In such apparatus according to the invention, an illumination-optical system is configured and situated to illuminate an xe2x80x9cillumination beamxe2x80x9d onto a desired region on a reticle defining a pattern to be transferred to a sensitive substrate (xe2x80x9cwaferxe2x80x9d). A projection-optical system is configured and situated to project, onto a desired corresponding region on the wafer, a xe2x80x9cpatterned beamxe2x80x9d created by passage of the illumination beam through the desired region on the reticle. A first beam-decelerating electric field is established between the reticle and the projection-optical system, and a second beam-decelerating electric field is established between the projection-optical system and the wafer. Aberrations arising from a convex lens action of the first beam-decelerating electric field and aberrations arising from a concave lens action of the second beam-decelerating electric field at least partially cancel each other. Thus, aberrations parasitic to the beam-decelerating electric fields are controlled relatively simply. (The lens actions of the first and second beam-decelerating electric fields can be opposite to what is described above.)
In a first representative embodiment of an apparatus according to the invention, the projection-optical system comprises a lens having a magnetic pole that is rotationally symmetric about an optical axis and an excitation coil. A liner tube is situated inside the inside diameter (ID) of the magnetic pole and its excitation coil. A first voltage (electrical potential) difference is imposed between the liner tube and the reticle, and a second potential difference is imposed between the liner tube and the wafer. These potential differences are utilized for forming the respective beam-decelerating electric fields.
In a second representative embodiment, an electron beam is used as the charged particle beam. The projection-optical system comprises a lens having a magnetic pole that is rotationally symmetric about the optical axis and an excitation coil. A first liner tube is situated inside the ID of the magnetic pole and its excitation coil. Similarly, the illumination-optical system comprises a lens having a magnetic pole that is rotationally symmetric about the optical axis and an excitation coil. A second liner tube is situated inside the ID of the magnetic pole and its excitation coil. A high negative voltage (Vk) is applied to a cathode of an electron gun (serving as the source of the electron beam), and a high positive voltage (Vi) is applied to the first liner tube (i.e., liner tube of the lens in the illumination-optical system). A high positive voltage (Vm), wherein Vmxe2x89xa7Vi, is applied to the reticle; a positive voltage (Vp), wherein Vp less than Vi, is applied to the second liner tube (i.e., liner tube of the lens in the projection-optical system). The wafer is either electrically grounded, or a voltage (Vw) is applied thereto, wherein Vw less than Vp.
In the second representative embodiment, not only can a high acceleration voltage (|Vk|+Vm) be established between the cathode of the electron gun and the reticle, but also the respective absolute values of Vk and Vm can be kept low. Also, a first beam-decelerating electric field can be realized between the reticle and the second (projection-lens) liner tube (Vmxe2x88x92Vp), and a second beam-decelerating electric field can be realized between the second (projection-lens) liner tube and the wafer (Vpxe2x88x92Vw, or Vpxe2x88x92O).
Desirably, |Vk|=|Vm|. Under such conditions, it is possible to minimize the absolute values of the voltage supplied by a power supply to the cathode and of the voltage (supplied by a power supply) applied to the reticle, while maintaining a high beam energy at the reticle.
It is also desirable that the energy of electrons incident to the reticle be at least 120 keV, and that the energy of electrons incident to the wafer be no greater than 60 KeV. By keeping the velocity of electrons of the beam incident to the reticle high, electron absorption by the reticle is reduced. Such reduction in electron absorption by the reticle results in less reticle heating during irradiation of the reticle. Reduced reticle heating reduces thermal deformation of the reticle, thereby increasing the accuracy of pattern transfer. In a scattering contrast method, reduced reticle heating increases the imaging contrast. On the other hand, reducing the velocity of electrons of the beam incident to the wafer reduces decreases in resist sensitivity and reduces heating of the wafer due to irradiation of the wafer, thereby increasing the accuracy of pattern transfer.
Any remaining aberrations can be corrected using multiple deflectors or a stigmator as required, provided between the reticle and the wafer. This further increases the accuracy of pattern transfer.
It is further desirable that the beam incident to the wafer have a beam half-angle (angle of a tangent to a lateral edge of the beam at the wafer relative to the optical axis) of at least 10 mrad. Directly upstream of the surface of the wafer, the patterned beam desirably is decelerated in the optical axis direction due to the action of the decelerating electric field between the projection-optical system and the wafer. However, because the patterned beam is not being decelerated in directions perpendicular to the optical axis, the beam half-angle becomes progressively larger with decreasing axial distance to the wafer. However, as the patterned beam propagates axially through the projection lenses (which are situated at a significant distance from the wafer), because the diameter of the patterned beam is small, space charge effects tend to increase. Such space-charge effects can be maintained at an acceptable level if the beam half-angle of the patterned beam incident to the wafer is at least 10 mrad (or desirably 12 mrad or more).
A contrast aperture desirably is provided between the reticle and the wafer at an axial position at which the axial distance from the wafer to the contrast aperture divided by the axial distance from the reticle to the contrast aperture is equal to the demagnification ratio of the projection-optical system. The distribution of beam intensity within the contrast aperture is desirably such that the off-axis beam intensity is greater than on-axis beam intensity. Such a beam is termed a xe2x80x9chollow beam,xe2x80x9d which further reduces space-charge effects within the projection-optical system.
The decelerating electric field(s) between the reticle and the wafer reduces the velocity of the charged particle beam in regions in which such electric fields are present. However, such beam slowing in the projection-optical system can result in increased distortion of the beam from space-charge effects. To avoid such a consequence, a combination of corrective measures desirably are employed such as using a hollow beam and using a relatively large beam half-angle at the wafer.
Each liner tube desirably comprises a ceramic, axially extended, cylindrical body of which the ID surface is coated with a film made of an electrically conductive material that extends over the entire ID surface and end surfaces. The outside diameter (OD) surface of the body is coated with a film made of an electrically conductive material that extends over the entire OD surface. A high voltage is applied to the ID film, while the OD film desirably is at ground potential. Because the OD film is thus at zero potential, no special insulation is required between the liner tube and components situated radially outside of the tube (such as the magnetic pole or excitation coil of the respective lens). Also, no special insulation is required between the magnetic pole and excitation coil of the respective lens and any component radially external to the lens.
Between the reticle and the liner tube of the projection lens and/or between the liner tube of the projection lens and the wafer (within the deceleration electric field/fields, respectively), an aperture plate desirably is situated that extends transversely to the optical axis. The aperture plate defines an opening that is rotationally symmetrical about the optical axis. The aperture plate is used, inter alia, to adjust the tilt of the electric field relative to the optical axis. In addition, charged particles of the beam incident to the aperture plate can be detected for purposes of, e.g., evaluating the patterned beam or for detecting alignment marks on the wafer or wafer stage. The aperture plate also can reduce aberrations due to non-planarity of the surface around the reticle or wafer.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.